1. Field of Invention
This invention relates to the use of electron irradiation and grinding along with an air classifier for the degradation of fluoropolymers, particularly polytetrafluoroethylene, to a uniform size and corresponding reduced molecular weight.
2. Description of the Prior Art
It is well-known in the prior art to use electron irradiation to degrade polytetrafluoroethylene (PTFE) so as to reduce its molecular weight resulting in a fine particle powder for use as a dry lubricant in paints and ink.
An early example of such a process is disclosed in U.S. Pat. No. 3,766,031 to Dillon wherein PTFE was exposed to electron irradiation. This process, however, was deficient in that there was no cooling apparatus and the PTFE was not irradiated uniformly.
The irradiation processes of the early prior art which irradiated the PTFE in trays were inefficient due to overscan of the trays, the gaps between the trays and the penetration characteristics of an electron beam. That is, the need to assure complete and uniform irradiation of a tray requires some overscan by the radiation and an efficiency loss of 5 to 15%. Likewise, there is usually some space between the trays of material which causes a further 10 to 15% loss of efficiency. However, the largest efficiency loss arises out of the fact that the dose received by the material varies with material depth. Typically, the dose at the surface is taken as the nominal dose for the material. Beam energy and/or material depth is adjusted so that an equal dose is effected at the opposite surface of the material. Radiation which passes entirely through the product is not utilized. Radiation in excess of the nominal dose is likewise not used. This causes further inefficiency, and in some instances may result in undesired properties of the resultant product. This depth-dose characteristic can cause processing inefficiency of up to 50%.
U.S. Pat. Nos. 4,748,005 and 4,777,192 issued to Neuberg and the instant inventor disclose water cooling, either with a water jacket or by direct spraying onto the PTFE, during electron irradiation of a selected portion of the PTFE while in a processing vessel. These processes result in a uniformly irradiated product. However, the uniformly irradiated product is of poor quality in that the resulting powder particles are of widely varying sizes and is not cost effective in that particles which are mechanically comminuted to a small size need less radiation than do other particles.
These references are designed for batch operation, which adds to the energy and economical inefficiency.
These references use a ribbon blender which requires large energy expenditures. Further, the apparatus of these references tended to fluidize PTFE. Additionally, the use of direct water cooling prevents proper degradation of the PTFE resulting in an inferior product. The use of direct water cooling also results in a great quantity of waste steam which contains fluorine gas, which combines with water to form hydrofluoric acid. Similarly, the use of a water jacket raises the concern that the water should be supplied at a temperature which is above the dew point of the surrounding air to avoid condensation. This may lead to the use of large quantities of water at elevated temperatures, and even the use of a water-to-water heat exchanger which adds to the complexity of the apparatus.